[ci] Update dependabot ignore list (#8864)

Updates the list of submodules that dependabot ignores when checking for
updates.  Repos which are no longer valid have been removed and recently
added xxhash repos have been added to this list.

.github/dependabot.yml

@@ -14,10 +14,8 @@ updates:
     schedule:
       interval: "daily"
     ignore:
-      - dependency-name: "external/apksig"
+      - dependency-name: "external/constexpr-xxh3"
       - dependency-name: "external/debugger-libs"
       - dependency-name: "external/lz4"
-      - dependency-name: "external/nrefactory"
-      - dependency-name: "external/opentk"
       - dependency-name: "external/robin-map"
-      - dependency-name: "external/sqlite"
+      - dependency-name: "external/xxHash"